target: merge release_cmd methods

The release_cmd_to_pool and release_cmd_direct methods are always the same.
Merge them into a single release_cmd method, and clean up the fallout.

(nab: fix breakage in transport_generic_free_cmd() parameter build breakage
 in drivers/target/tcm_fc/tfc_cmd.c)

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>